1. Field
The present disclosure relates to a modified electrically conductive adhesive (MECA) formed by mixing the surface-modified conductive fillers and resins. The surface-modification of the conductive fillers can result in formations of a thin layer containing nano-structures of the partially oxidized metal halides or pseudohalides at the surface of the fillers, which can affiliate the electrical conductivity among the conductive fillers after curing the MECAs.
2. Background
Efforts have been made to replace lead-containing solder materials with lead-free conductive adhesives in surface mount technology. The lead-free adhesives have unique mechanical properties, processing properties, thermal properties, and reliability, and they are environmental friendly. Resin-based polymers are used as matrices for preparing electrically conductive adhesives (ECAs) for applications such as electrical joints between a printed circuit board and the surface mount components. FIG. 1 is a SEM photomicrograph of an ECA. The ECA consists of a micropattern of adhesive resin and conductive fillers. In FIG. 1, the fillers appear as the darker portions. Referring to the figure, the dispersant is lighter in color than the silver because of the charging effect. The conductive fillers with a certain loading ratio are responsible for the electrical interconnection while the resin mainly provides the mechanical interconnection. Typically the silver microflake weight ratio is over 75% in a traditional ECA formulation. By way of example, a commercial sample (Epotek H20E) is 85 wt %. Compared with the eutectic Sn/Pb solders, a disadvantage of ECAs is the lower electrical conductivity, thus resulting in poor current carrying capability. ECAs are primarily composed of two parts: conductive fillers, such as silver, gold, or copper, etc., and a resin matrix. Since the resin matrix is electrically insulative, the overall electrical resistance is the sum of the resistance of fillers, the contact resistance among fillers and between fillers and pads. Therefore, improving the contact efficiency and the evenness of dispersion can be two primary strategies to improve the electrical conductivity of the ECAs.
The dimension of the conductive fillers is one of the key factors influencing the conductivity of the ECAs. Micron-sized flake shaped fillers, such as silver micro-flakes, showed excellent performance in electrical conductivity in previous work. Nano-sized filler often results in higher bulk resistivity of the ECA, which is due to the increased number of contact points between filler particles and the consequently increased contact resistance. Smaller fillers can improve the isotropic character in the resin matrix, which is especially useful in the area of high-resolution fine-pitch interconnects.
Electrical percolation is used to describe the conductive property of a composite material which includes materials which may have substantially different properties. In the case of ECAs, the composite material behaves like a matrix polymer but above a threshold concentration, the composite material behaves more like the conductive filler. This ability of the composite material to behave substantially like the filler is called electrical percolation. According to the theory of electrical percolation, the change is the result of the decrease in distance between filler particles and the resultant ability of the particles to interact with each other as electrically charged particles. This is achieved by establishing an active conductive pathway through the conductive filler.
The electrical percolation threshold pc is a critical value related to the loading density or concentration of the nano-structures, above which long range connectivity can be achieved. Conducting filler-insulating polymer composites become conductors when the filler content reaches a critical value, or threshold percolation, characterized by a sharp increase of the electrical conductivity. The percolation threshold represents a transition from a local to an infinite conducting state.
Typical filler loading for an ECA formulation is over 25 volume percent. Within this range, the materials exceed an electrical percolation threshold and are electrically conductive in all directions after the materials are cured. An example of such a filler is a filler formed with micron-sized silver flakes. Previous researchers studied the surface modification of the fillers using organic surfactants (e.g., stearic acids, C-18 carboxylic acids) in enhancing the similarity of the polarity between the fillers and polymeric matrixes. These surfactants can reduce the viscosity of the conductive adhesives and prevent agglomerations of the inorganic fillers; however, there is a concern that the surfactants decrease the conductivity of the ECAs due to the insulative property of the surfactants. Some attempts have been made to enhance the conductivity by modifying the filler surfaces using aldehydes or carboxylic acids, which have the potential to reduce the adverse affect of oxidation of the inorganic filler materials; however, considering the low conductivity nature of these organic molecules, the improvement is limited. Other attempts have been made to use smaller (nano-sized) filler materials to replace a portion of the micron-sized fillers in order to decrease the sintering temperature of the ECAs; however, this method may result in an increment in contact resistivity because of the increased number of the contact points.
The present subject matter relates to the surface modification of the micron-sized filler materials which are applied in ECAs. The nano-structures formed at the filler surface can alter the wettability and oxidation property of the metal fillers. Different to any other work ever reported, the method presented here can significantly improve the percolation threshold of the modified ECAs by decreasing the use of the conductive fillers down to 20˜30 wt %.